This invention relates generally to optical communication systems and, more particularly, to a method and system for actively reducing polarization mode dispersion along an optical communications link.
In a typical optical communications system, an optical signal in the form of a series of light pulses is emitted from an optical transmitter and coupled into an optical fiber through which the optical signal may propagate for hundreds of miles. The optical transmitter typically comprises a laser diode that is intensity modulated with a data signal so that the optical signal can transmit information over long distances through the fiber.
A sensitive receiver at the opposite end of the fiber can detect the pulses in the optical signal and reconstruct the data signal that was applied at the transmitter. The data signal may be, for example, a SONET-compliant STS-48 digital signal carrying data at a rate of about 2.5 gigabits-per-second. (SONET refers to the Synchronous Optical NETwork standards that define particular digital signal formats.) As this type of signal modulates an optical transmitter, a modulated optical signal called an OC-48 is formed comprising a train of closely spaced light pulses of very short duration. A single pulse for this type of signal has a duration of less than a billionth of a second. Other types of signals with several times the data rate of an OC-48 are becoming commonplace.
As these brief pulses propagate through a long optical fiber, a number of effects cause each of the pulses to spread out in the time domain. Without sufficient control of these effects, the pulses can even begin to overlap such that a receiver cannot distinguish one pulse from another and therefore cannot reliably reconstruct the transmitted signal. Many techniques have been developed for reducing or eliminating large-magnitude degradation effects that occur in optical fibers, such as multi-mode propagation and chromatic dispersion. But another form of dispersion is becoming a limiting factor in optical communication systems as progressively higher data rates are attempted.
Polarization mode dispersion (PMD) arises due to birefringence in the optical fiber. This means that different polarizations of light may propagate at slightly different speeds through a given optical fiber. A brief pulse of light may have a well defined on-off profile upon entry into an optical fiber. But, unless some control means are employed, the energy of the pulse will probably be partitioned into polarization components that travel at different speeds. As with chromatic dispersion, this speed difference causes pulse broadening and restricts the usable bandwidth of each modulated optical signal.
A modulated optical signal arriving at an optical receiver must be of sufficient quality to allow the receiver to clearly distinguish the on-and-off pattern of light pulses sent by the transmitter. Conventionally, a properly designed optical link can maintain a bit-error-rate (BER) of 10xe2x88x9213 or better. This means that, on average, one bit will be incorrectly communicated for every 10,000,000,000,000 bits sent. Noise, attenuation, and dispersion are a few of the impairments that can render an optical signal marginal or unusable at the receiver. Generally, when an optical channel degrades to a BER of 10xe2x88x928, a communication system will automatically switch to an alternate optical channel in an attempt to improve the BER.
From the receive end of an optical transmission link, it is difficult to observe a signal and determine what effects are contributing to the degradation of the signal. Chromatic dispersion causes the spreading of pulses in much the same manner as polarization-mode dispersion. Fortunately, chromatic dispersion is relatively constant for a given fiber and can be measured or calculated when a fiber is initially installed and can be adequately compensated by static techniques.
Although PMD is a more subtle effect, PMD changes very dramatically as a fiber is moved about or subjected to physical stress or thermal changes. A fiber installed above ground can exhibit fairly rapid fluctuations in PMD due to temperature and mechanical forces. A fiber buried underground may be sensitive to loads such as street traffic or construction work. The state of polarization of a signal may be affected at any point along the fiber by such influences but the precise effect that will be experienced is practically unpredictable for a long fiber. Consequently, a fiber that is carrying a high data rate optical signal may present an acceptable path at one time and a severely degraded path at another time. Accordingly, there is a need to dynamically compensate PMD such that, as the propagation characteristics of a fiber change, some accommodation can be made to maintain an acceptable path for the optical signal. This can be especially difficult as the fiber must continue to carry traffic at all times. Traffic along the fiber can not be interrupted to allow for test signals to be applied to the fiber and for experiments to be performed.
Generally, for a given fiber at a given instant in time, it is possible to launch an optical signal with a particular state of polarization such that birefringent effects tend to be nearly eliminated. Polarization controllers are known in the art for changing the state of polarization of an optical signal. Although one may place a polarization controller at the point where an optical signal is launched into a given fiber, it is not known what polarization should be selected to best mitigate PMD through the fiber at any point in time. As an added challenge, a reliable means must be used for communicating to the polarization controller even when the PMD conditions along the fiber are extreme. This implies that a second communications link, separate from the given fiber, is preferable for signaling to the polarization controller. However, this approach adds cost and adds the control complexity of ensuring that the correct polarization controller is addressed from among many polarization controllers throughout the network.
In prior art optical communication systems, PMD changes are typically compensated for by a Polarization Mode Dispersion Compensator (PMDC) which detects the differential delay experienced by two polarizations of an optical carrier and then adaptively corrects the delay. A PMDC is generally a self-contained unit placed along an optical link just prior to a receiver. As polarization characteristics of the fiber change, the PMDC constantly monitors and adjusts the signal in an attempt to minimize the PMD contribution to overall dispersion. A typical PMD compensator splits an incoming modulated optical signal into two polarizations. The relative timing of the two modulated signal halves is then corrected by introducing a delay into one signal half and then recombining the halves to form a corrected output signal. This compensation technique has practical limitations to the range of PMD values that it can detect and correct.
An improved means is required for actively compensating PMD along an optical communications link. In particular, a method is required for directing a polarization controller to change the polarization of an optical signal so that the signal experiences reduced PMD as it propagates through an optical fiber. Furthermore, a means is required for detecting and compensating PMD regardless of the magnitude of the PMD.
The present invention is directed to an optical communications link having improved control of PMD characteristics. The quality of a first optical signal received through a fiber is measured by a receiver. The resulting quality measurement is used to determine how to adjust a polarization controller to change the polarization of the first optical signal as it is transmitted into the fiber before reaching the receiver. A command signal to control the polarization controller is generated based upon the quality measurement and is coupled to the polarization controller to achieve an optimum polarization setting.
In accordance with a preferred embodiment, the command signal for the polarization controller is sent back along the same fiber through which the first optical signal is received.
In accordance with one exemplary embodiment of the present invention, the quality measurement is an observed bit-error-rate from the receiver.
In accordance with another exemplary embodiment of the present invention, the quality measurement is a theoretical best-attainable BER extrapolated from measurements at various settings of threshold and sampling time offset in the receiver.
In accordance with yet another exemplary embodiment of the present invention, the quality measurement is an assessment of PMD derived from measurements at various settings of threshold and sampling time offset in the receiver.
Various exemplary embodiments are also taught by which control signals from the receiver are coupled to the polarization controller. In accordance with a preferred embodiment of the present invention, the control signal is communicated in the form of low-level, low frequency modulation superimposed upon a second modulated optical signal. The second modulated optical signal travels along the same fiber that carries the first optical signal, but in a direction opposite to the first optical signal.